A solid polymer electrolyte fuel cell includes a solid electrolyte membrane, such as a perfluorosulfonic acid membrane, acting as an electrolyte, and a fuel electrode and an oxidizing electrode that are bonded to the respective sides of the solid electrolyte membrane. The solid polymer electrolyte fuel cell generates power by electrochemical reaction while a liquid organic fuel, such as hydrogen or methanol, is being supplied to the anode and oxygen is being supplied to the cathode.
Among fuel cells, a type represented by a direct methanol fuel cell has been actively developed in which a liquid organic fuel is directly supplied to the fuel electrode without being reformed. Since this type of fuel cell is designed so that the liquid organic fuel is directly supplied to the fuel electrode, no device such as reformer is required. Accordingly, the cell advantageously can be simple in structure and can be downsized. In addition, the liquid organic fuel, which is used as the fuel in this type of fuel cell, can be more easily and safely delivered than gas fuels, such as hydrogen gas and hydrocarbon gases.
In the fuel cell, electrochemical reactions occur at the respective electrodes. If, for example, methanol is used, the anode causes the reaction:CH3OH+H2O→6H++CO2+6e−  [1]The cathode causes the reaction:3/2O2+6H++6e−+3H2O  [2]In order to induce these reactions, each electrode is made of a mixture of carbon particles supporting a catalyst and a solid polymer electrolyte.
If such a solid polymer electrolyte fuel cell uses methanol as the fuel, the methanol supplied to the anode (fuel electrode) reaches the catalyst through the pores in the electrode. Then, the methanol is decomposed by the catalyst to generate electrons and hydrogen ions as expressed by the above reaction formula [1]. The hydrogen ions reach the cathode (oxidizing electrode) via the electrolyte in the anode and across the solid electrolyte membrane between the two electrodes, and react with oxygen supplied to the cathode and electrons coming from an external circuit to produce water as expressed by the above reaction formula [2]. On the other hand, the electrons released from the methanol are extracted to the external circuit via the supported catalyst in the anode, and flow into the cathode from the external circuit. Consequently, electrons flow in the external circuit from the anode to the cathode, and power is extracted.
The direct methanol fuel cell using methanol as the fuel has a large possibility of being used as a portable small-size fuel cell, and is accordingly actively developed as a next-generation secondary battery used for, for example, portable computers and cellular phones.
In general, a fuel cell using a liquid organic fuel includes a solid polymer electrolyte membrane made of a solid polymer ion-exchange resin as the electrolyte. To ensure fuel cell functions, in this instance, it is necessary that the hydrogen ions generated from the fuel electrode (anode) transfer to the oxidizing electrode (cathode) across the membrane. It is known that the transfer of hydrogen ions is accompanied with the transfer of water, and the electrolyte membrane needs to contain a certain amount of water.
In use of a liquid organic fuel with high affinity to water, such as methanol, however, the liquid organic fuel diffuses in the solid polymer electrolyte membrane containing water, and finally reaches the oxidizing electrode. This phenomenon is called crossover. The crossover causes the liquid organic fuel, which is originally intended to provide electrons to the fuel electrode, to transfer to the oxidizing electrode, thereby oxidizing the liquid organic fuel. The resulting fuel cannot be used effectively, and accordingly causes the fuel cell to reduce the voltage or power and the fuel efficiency. The crossover of methanol becomes more pronounced as methanol concentration in the fuel is increased. It is therefore difficult that the direct methanol fuel cell uses methanol fuel at high concentrations.
In order to overcome the problem with methanol crossover, a variety of approaches have been proposed as follows:
[1] A method of improving the electrolyte membrane between the anode and the cathode and its structure (Japanese Unexamined Patent Application Publication Nos. 11-26005, 2002-83612, etc.).
[2] A method of supplying a liquid fuel to the anode, and in which the liquid fuel is vaporized by a vaporizer or heater and then supplied (Japanese Unexamined Patent Application Publication No. 2001-93541, for example).
[3] A method of replacing methanol with any other alcohol (isopropanol, Japanese Unexamined Patent Application Publication No. 2003-217642) or any other organic fuel (cycloparaffins, Japanese Unexamined Patent Application Publication No. 2003-323896).
[4] A method of suppressing the crossover by adding a sugar, an alcohol, or the like to the liquid organic fuel and thus using osmotic pressure in the fuel cell (Japanese Unexamined Patent Application Publication No. 2004-39293).
These methods however do not sufficiently overcome the methanol crossover, and further improvement is required.
In addition to the methanol crossover, the direct methanol fuel cell using methanol as the fuel has other problems:
(1) Undiluted methanol is designated as a deleterious substance under Japanese Poisonous and Deleterious Substances Control Law and classified in hazard category 4, and therefore should be handled with due care; and
(2) Since methanol is liquid, an airtight container preventing leakage is required.
These two are problems in handling methanol.
(3) If a methanol aqueous solution with a high concentration is used to enhance the power generation efficiency, the electrodes of the fuel cell can be damaged or the peripheral metals may be corroded.
In addition, the following problem may occur.
(4) Methanol used as the fuel is generally in a form of aqueous solution of about 10% to 30% by weight. This is because the methanol crossover becomes pronounced as the methanol concentration is increased; because undiluted methanol is designated as a deleterious substance under Japanese Poisonous and Deleterious Substances Control Law and classified in hazard category 4, and therefore should be handled with due care; and because high concentration methanol disadvantageously causes corrosion or the like. However, a methanol aqueous solution of such a low concentration may disadvantageously freeze particularly in cold climates. The frozen methanol solution cannot be used as the fuel, and needs to be unfrozen before use. Also, the methanol solution disadvantageously produces concentration distributions in methanol and water during being frozen. In order to eliminate the concentration distributions, the frozen fuel needs to be unfrozen completely, and is then homogenized by, for example, shaking the container. Thus, methanol is complicated to use.
Although other organic fuels than methanol can produce the same problems (1) to (4), the problems are not solved in the known fuel cells.
In addition, there has been no method for easily detecting the remaining quantity of the fuel cell fuel. Accordingly, it is difficult to prepare a necessary amount of fuel or to supply the fuel appropriately before it is completely consumed so that the fuel is supplied at appropriate timing.
Furthermore, known fuel cells have the following problems.
Hydrogen and methanol, which are used as fuel cell fuels, are dangerous and have many problems in handling. In order to store such fuels safely, methods have been disclosed of turning the fuel into a molecular compound (WO2004000857) or of gelate the fuel by allowing a polymer to absorb the fuel (Japanese Unexamined Patent Application Publication No. 2004-127659). In order to release the fuel cell fuel from such a stable composition prepared from the fuel cell fuel, in general, the fuel composition is heated.
However, heating the fuel composition containing the fuel cell fuel to release the fuel requires a heating apparatus and high energy for heating. This is industrially disadvantageous. A simple method is desired for releasing a fuel from a fuel composition containing a fuel cell fuel.